In some engineering field such as construction and decoration, horizontal or perpendicular reference lines are often needed to facilitate other operations. Chinese Patent No. 96209131.6, with a title of “Three-dimensional Optical Levelling, Plumbing and Angle-calibrating Instrument”, disclosed a self-leveling instrument which comprises a pendulum universally mounted on the frame for free swinging thereabout, laser generators and lens assembly mounted on the pendulum thereby to form a horizontal and a perpendicular laser reference lines on the object surface. Obviously it is facile to use this kind of self-leveling laser instrument comparing to manually line-marking. While new problems are occurred accompany with the self-leveling instrument. Since the pendulum is free swinging all the time, this may induce unexpected damage or loss of precision to the laser generator and lens assembly according to irregularly swinging of the pendulum during transportation or other movement of the leveling instrument, and thereby the use life of the instrument is reduced.